This invention is in the field of dot matrix printers and more particularly concerns a method and apparatus for improving the effective resolution of such a printer.
The invention will be described in relation to thermal ink jet dot matrix printers which, due primarily to frequency response limitations, have a particular maximum useable horizontal dot placement resolution. Many low cost thermal ink jet printers, for example, have resolutions of 300 dots per inch.
Thermal ink jet printers generate printed characters or graphics on a sheet of paper or other print medium by placing dots on a fixed matrix grid with a predetermined horizontal and vertical resolution. Many thermal ink jet printers in use today have a maximum resolution of 300 dots per inch (dpi) vertical by 300 dpi horizontal.
Some such printers store font data on a 300 dpi vertical by a pseudo 600 dpi horizontal grid to improve text print quality. While this approach does improve print quality for some text modes, it does nothing for graphics and text received in a bit-image mode and doubles the amount of memory required to store the font data because of the increased horizontal resolution.
Laser printers, which may also be considered dot matrix printers, have developed a number of resolution enhancement techniques. FIGS. 1 and 2 show techniques which are used to improve the print quality of a 300 dpi laser printer. In the illustrated example, in a 300 dpi laser printer, one type of "near vertical" (oblique or diagonal) line is approximated by staggering groups of four vertically stacked dots placed on a horizontal 1/300 inch grid. The dots are on a 1/300 inch grid because the maximum horizontal resolution of the data, either stored in memory or being received from the attached printer is 300 dpi. FIG. 1A shows this situation. FIG. 1B shows the printed result after the data has been post processed by smoothing hardware inside the laser printer. This hardware compares the incoming data with pre-stored templates, matching the best template with the incoming unsmoothed data. The template selected directs the printer control hardware to vary the start/finish of a laser flash on a 1/2400 inch grid. Once the smoothing hardware finds a match between incoming data and a template, it simply follows the laser flash control designated by the template to obtain the desired application of toner to obtain the best possible smoothing given the 1/2400 inch flash control. This method is also used to smooth near vertical black/white boundaries in the same fashion.
An example of smoothing of a "near horizontal" line is shown in FIG. 2. In this case the near horizontal line is approximated by four horizontally adjacent dots placed on a 1/300 inch vertical grid, since 1/300 inch is the maximum vertical resolution of the stored or received data. This situation is illustrated in FIG. 2A. In FIG. 2B, the incoming data is again compared to the stored templates until a match is found. The matching template then directs the laser control to vary the start/stop of the laser flash. The closer the time between the start and stop of the flash (the flash width), the smaller the toner dot that attaches to the drum. Also these smaller dots of toner are attracted to one another in the vertical direction. This attraction which causes the dots to tend to merge in the vertical axis is fundamental to obtaining smoothing of near horizontal lines or near horizontal black/white boundaries because it yields an ability to shift dot position on the vertical axis in a finer increment than 1/300 inch.
It is an objective of the present invention to describe a print quality enhancement method and apparatus which improves the apparent resolution of a printer such as a thermal ink jet dot matrix printer with a maximum horizontal resolution of some specified amount. In the embodiment of the invention to be described in detail hereinafter, the horizontal resolution is 300 dpi. The improvement to this resolution results in improved print quality performance in the printing of text characters and graphics that are oriented off of the vertical axis (near vertical features), such as in oblique text like italics. Correction for near horizontal features is not included because of the difficulty in ink jet technology of modulating the dot size and getting the dots to merge together, as was shown in the earlier explanation of near horizontal correction in laser printing. To obtain some degree of smoothing of near vertical print features only requires that the printed dot be shifted on a finer horizontal grid (such as 600 dpi) than the printer's specified maximum (for example 300 dpi). Spot size modulation and spot merging as employed in laser printer resolution enhancement are not required for this. Correcting only for near vertical features also reduces the number of templates required by roughly half. Also, when printing text, near vertical conditions occur more often than near horizontal, such as in italic printing, and the print quality defects of near vertical features in text tend to be more apparent to the observer.
In order to produce image enhancement as described, the bit image data created has a vertical resolution of 300 dpi and a horizontal resolution of 600 dpi. The resultant bit map, if handled prior to printing, would require twice the storage and processor bandwidth as the original data. It is therefore a further objective of the invention to provide a print quality enhancement method and apparatus which eliminates the need for such extra hardware and software to handle this larger data set.
In accordance with one aspect of the invention, this need for extra hardware and software is eliminated by creating and considering only a limited number of the high resolution data pels at a time. In the illustrated form of the invention, the number of pels of high resolution data processed to eliminate unprintable dots is six. Using a traveling six pel window, together with two other control inputs, the system can intelligently modify the data such that it is printable by a lower resolution device.
For example, the maximum fire rate of the print nozzles combined with the target speed of the print head carrier allows consecutive dots to be placed a minimum of 0.0033 inches apart on one type of printer embodying the invention. This results in a maximum printable horizontal resolution of 300 dpi. The encoding scheme on the printer allows for 600 dpi addressability. In other words, the printer is capable of placing the pels on a 600 dpi grid. The above-mentioned limitations of the print head speed and nozzle firing rate imply that no two consecutive 600 dpi pels can be printed. Since the enhanced print data has a horizontal resolution of 600 dots per inch, some of the dots must be eliminated before the data is sent to the print head.
A simple combination of each pair of dots from the data stream, while satisfying the firing criteria for the print head, will defeat the purpose of the original print enhancement. When print enhancement produces odd strings of 600 dpi pels, an exact representation of the 600 dpi image is not possible. An intelligent algorithm for synthesizing the data can both satisfy the print head requirements and maintain the improved edge definition obtained through the print enhancement. An aspect of the invention, when implemented in the printer hardware, will process the enhanced data "on the fly" while remaining transparent to the printer software.